1. Field of the Invention
The present invention relates to an object detecting device and, more particularly, to an object detecting device whereby the number and size of detected objects can be determined.
2. Description of the Relevant Art
In recent years, systems for vehicles to which a radar technique is applied have been enthusiastically developed. For example, a driver support system wherein the distance between one's vehicle and the vehicle ahead and the like are measured using a radar so that a safe distance therebetween is kept based on those measurements, and a preventive safety system for avoiding a collision and the like by detecting the presence of a pedestrian or the like in advance using a radar, have been known.
FIGS. 8(a) and 8(b) are illustrations showing an object detecting operation by a conventional radar, wherein FIGS. 8(a) and 8(b) show the radio wave reception status in the case of multiple small objects being present ahead of a vehicle and that in the case of a large object being present ahead thereof, respectively.
Reference numeral 1 in the figure represents a radar, being installed forward of a vehicle M, which measures the distance between the vehicle M and an object present ahead thereof and the like. The radar 1 comprises a transmitter-receiver 2 for transmitting an electromagnetic pulse through an antenna (not shown) arranged in such a manner as to point in the forward direction of the vehicle M and receiving through the antenna a reflected pulse of the electromagnetic pulse so as to detect the time (so-called delay time) between transmission of the electromagnetic pulse and reception of the reflected pulse, and a processing element 3 for computing the distance to an object and the like based on the delay time detected by the transmitter-receiver 2. Using the radar 1, the distance to the nearest object to the vehicle M (or the shortest distance) is obtained based on the delay time.
However, when there are multiple small objects (A and B) ahead of the vehicle M as shown in FIG. 8(a), only the distance from the vehicle M to the object A closest thereto is detected by the radar 1. As a result, even if another object B is present still ahead in the periphery of the object A, the presence of the object B cannot be detected. In addition, the size of the object A cannot be determined.
When there is a large object C present ahead of the vehicle M as shown in FIG. 8(b), the shortest distance between the vehicle M and the object C is detected using the radar 1, but the size of the object C cannot be determined.
FIGS. 9(a) and 9(b) are illustrations showing the object detecting operation by a conventional dual-type radar. And FIGS. 9(a) and 9(b) are illustrations showing the radio wave reception status in the case of multiple small objects being present ahead of a vehicle and that in the case of a large object being present ahead thereof, respectively.
A radar 1A comprises transmitter-receivers 2A and 2B each for receiving a reflected pulse of an electromagnetic pulse transmitted by itself so as to detect the time (delay time) elapsed before receiving the reflected pulse, and a processing element 3A for computing the distance to an object and the like based on each delay time detected by the transmitter-receivers 2A and 2B, being installed forward of a vehicle M. Using the radar 1A, the distances from each of the transmitter-receivers 2A and 2B to the nearest object to the vehicle M are obtained based on the delay times detected by the transmitter-receivers 2A and 2B (the times each elapsed before receiving the reflected pulse of the electromagnetic pulse transmitted by itself), respectively, and the direction in which the detected object is present is obtained (here, in cases where the same object is detected by the transmitter-receivers 2A and 2B).
When there are small objects A and B present ahead of the vehicle M as shown in FIG. 9(a), the object A is detected by the transmitter-receiver 2A while the object B is detected by the transmitter-receiver 2B in the radar 1A, so that the shortest distance between the transmitter-receiver 2A and the object A and that between the transmitter-receiver 2B and the object B are detected, respectively. However, the radar 1A erroneously detects only one object being present at the point of intersection of an arc A1 with the distance from the transmitter-receiver 2A to the object A as radius and an arc B1 with the distance from the transmitter-receiver 2B to the object B as radius.
When there is a large object C ahead of the vehicle M as shown in FIG. 9(b), the shortest distance between the transmitter-receiver 2A and the object C and that between the transmitter-receiver 2B and the object C are detected, respectively, in the radar 1A. The radar 1A erroneously detects only one object being present at the point of intersection of an arc C1 with the shortest distance from the transmitter-receiver 2A to the object C as radius and an arc C2 with the shortest distance from the transmitter-receiver 2B to the object C as radius, so that the radar 1A erroneously judges the object C as being present nearer than it really is. In addition, the size of the object C cannot be determined.
When the direction of radio waves transmitted from a transmitter-receiver is thus fixed, the number and size of detected objects can be determined neither in the case of a single transmitter-receiver nor in the case of dual transmitter-receivers.
On the other hand, Japanese Kokai No.1994-148329 teaches that by transmitting radio waves in a laterally sweeping manner and receiving a reflected wave at every sweep angle so as to detect angle and distance data of an object to a vehicle, the size of a detected object can be obtained from the angle and distance data. It is possible to detect the direction in which an object is present, the size and number thereof and the like by thus sweepingly transmitting radio waves. However, in such device, a unit for sweepingly transmitting radio waves is additionally required. Furthermore, the size of an antenna becomes larger in order to increase its directivity and the construction of the device becomes more complex, resulting in a higher cost of components.